1. Field of the Invention
This invention relates to a radiation image read-out system for reading out a radiation image such as a radiograph recorded in a stimulable phosphor stored with radiation energy in an image-like pattern. More particularly, the present invention relates to a method of reading out a radiation image recorded in a stimulable phosphor which is used in a radiation image recording and reproducing system utilizing stimulability of a phosphor, and an apparatus for carrying out the method.
2. Description of the Prior Art
Instead of the conventional photographic system used in the medical radiography, it has been recently developed to use a stimulable phosphor as a recording medium in which a radiation image as of X-rays is first recorded by irradiating the phosphor with a radiation through an object like a human body and then stimulating the phosphor with stimulating rays like visible rays or infrared rays to cause the phosphor to emit detectable light in the pattern of the recorded image. More concretely, as shown in U.S. Pat. No. 3,859,527, a stimulable phosphor is exposed to an imagewise radiation of X-rays or the like to store or record a radiation image through an object in the form of a latent image, and then an stimulating or exciting light beam is caused to scan the stimulable phosphor to have the phosphor stimulated and emit visible light in the form of the recorded pattern. The scanned phosphor is detected by a photodetector to detect the emitted light along the scanning line. The output of the photodetector is used as an image signal to display or reproduce the recorded image in the form of a visible image.
On the other hand, it has also been known in the art in place of the conventional radiography to use a thermoluminescent phosphor as a recording medium and record a radiation image as a latent image thereon and read out the recorded image by exciting the latent image by use of heat rays. This kind of radiation image recording and reproducing system is shown in British Pat. No. 1,462,769 and Japanese Patent Laid Open No. 51 (1976)-29889. The thermo-luminescent phosphor stored with a radiation image emits light when excited by the heat rays, and the emitted light is detected by a photodetector to obtain an image signal. This system which uses thermo-luminescence is disadvantageous in that the recording medium is required to have heat resistance. In view of the required heat resistance, there are severe limitations in the kind of the adoptable phosphor and the substrate on which the phosphor is applied.
The present invention is concerned with the former type radiation image recording and reproducing system and particularly relates to a read-out method for this system. The inventors of this invention made various investigations regarding efficiency, speed and signal-to-noise ratio in the above radiation image read out process utilizing the stimulable phosphor. As a result, the following phenomena have been discovered.
(1) The efficiency of stimulation greatly depends upon the wavelength of the stimulating rays when a time has passed since recording of the radiation image. In other words, the lowering of the efficiency of stimulation or output luminance of the stimulated phosphor which is considered to be due to decay of the stored energy depends upon the wavelength of the stimulating rays used for stimulating the phosphor. In more detail, when a period has passed since recording of the radiation image, the level of output luminance of the stimulated phosphor changes with the wavelength of the stimulating rays.
(2) The response or speed of stimulation depends upon the wavelength of the stimulating rays. The speed of stimulation greatly effects on the speed of read-out of the recorded radiation image.
(3) The signal-to-noise ratio in the read-out step is greatly effected by the reflection light of the stimulating rays reflected by the surface of the phosphor and the ambient light, since the intensity of the stimulated emission is very small.